This invention concerns improvements in or relating to electrochemical, amperometric gas sensors, for example toxic gas sensors or oxygen sensors of the kind disclosed in UK Patents Nos 1,571,282 and 2,094,005.
It is common practice in the field of such sensors to incorporate gas controlling gas diffusion barriers employing capillary hole barriers, solid membrane barriers or Knudsen barriers, and one of the attendant potential problems associated therewith is their blockage through contamination. For example, in industrial environments it is not unusual for diffusion barriers to become fouled or filled by overpainting, with water through washing down, with mud or other extraneous performance-inhibiting matter. Such fouling is not uncommon and it has been known for instruments to be dropped down manholes or into pits for example where they encounter water or mud with the damaging consequences indicated above.
The blocking of a diffusion barrier obviously affects the performance of the sensor. In the case of an oxygen sensor, the effect is that it fails safe by virtue of a decay in the signal resulting from oxygen starvation and accordingly a low oxygen alarm will be given. In complete contrast, however, with a toxic gas sensor fouling of the diffusion barrier will result in a fail dangerous situation since any toxic gas in the ambient air will no longer be able to access the sensor and will not therefore be detected. Examples of toxic gases which are commonly detected with electrochemical amperometric gas sensors are, carbon monoxide, hydrogen sulphide, sulphur dioxide, nitric oxide, nitrogen dioxide, chlorine and others.
An object of the present invention is thus to provide an improved sensor for detecting both toxic gas and oxygen levels which obviates the disadvantage attaching to conventional sensors.
According to the present invention, in a combined toxic gas sensor and an oxygen sensor including diffusion barriers, the oxygen sensor is mounted such that its diffusion barrier communicates with the downstream side of the diffusion barrier of the toxic gas sensor.
The diffusion barriers are of the capillary hole type as a preference, but it is to be understood that other types of barrier, for example solid membrane barriers or Knudsen barriers, may be employed. It is to be understood that the diffusion barrier of one of the sensors may be different from that of the other sensor.
In this arrangement the oxygen sensor is so placed as to monitor the oxygen concentration within the region between the toxic gas sensor diffusion barrier and its sensing electrode. The toxic gas diffusion barrier will normally present no significant restriction to the ingress of oxygen from the ambient air and the oxygen sensor can function in its normal way to detect oxygen deficiency in the ambient air.
In practice, there will be some reduction in the oxygen signal given by the relationship:       1    S    =            1              S        ⁢                  xe2x80x83                ⁢        O              +          1              S        ⁢                  xe2x80x83                ⁢        T            
where S is the oxygen sensor signal in position on the toxic gas sensor, SO is the oxygen sensor signal with its diffusion barrier open to the air and ST would be the oxygen sensor signal inside the toxic gas sensor without its own controlling barrier. ST is usually very much greater than SO and therefore S tends to SO. However, even if ST were a significant series diffusion barrier compared to SO, the oxygen sensor signal could be easily calibrated in the assembled condition and the toxic gas sensor barrier check and oxygen deficiency functions would operate as described for this invention.
In the event that the toxic sensor diffusion barrier becomes blocked, the oxygen sensor will consume any oxygen within the internal space of the toxic sensor. As a result the oxygen sensor signal will decline and eventually an oxygen deficiency alarm will be initiated. In order to differentiate unambiguously between a blocked diffusion barrier on the toxic gas sensor and a genuine oxygen deficiency condition in the ambient environment, a second oxygen sensor with its capillary diffusion barrier open directly to the external environment may be provided. However, in the absence of a second oxygen sensor, a fail-safe alarm will be given in the event of either a low oxygen condition or a blocked diffusion on the toxic gas sensor.
Any suitable means may be provided for securing the sensors together to form a modular unit.